1. Tracking processes: top

Understanding how to read and analyze resource utilization is a critical skill for any Linux user. Many consider mastering command-line tools the best way to learn Linux from the ground up, as they provide deep insight into how Linux works. This lesson introduces top, a powerful utility for tracking what your processes are doing in real-time.

Understanding the top Command

We have briefly mentioned top before, but now we will dig into the specifics of what it displays. The top command gives you a dynamic, real-time view of the processes and system utilization on your machine.

top - 18:06:26 up 6 days,  4:07,  2 users,  load average: 0.92, 0.62, 0.59
Tasks: 389 total,   1 running, 387 sleeping,   0 stopped,   1 zombie
%Cpu(s):  1.8 us,  0.4 sy,  0.0 ni, 97.6 id,  0.1 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:  32870888 total, 27467976 used,  5402912 free,   518808 buffers
KiB Swap: 33480700 total,    39892 used, 33440808 free. 19454152 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
 6675 patty    20   0 1731472 520960  30876 S   8.3  1.6 160:24.79 chrome
 6926 patty    20   0  935888 163456  25576 S   4.3  0.5   5:28.13 chrome

Let's go over what this output means. You don't have to memorize this, but you can use this lesson as a reference.

System Summary

The first few lines provide a high-level summary of the system's state.

• 1st line: This is the same information you would see if you ran the uptime command. It shows the current time, system uptime, number of logged-in users, and the system load average over the last 1, 5, and 15 minutes.
• 2nd line: A summary of all tasks (processes), categorized as running, sleeping, stopped, or zombie.

CPU Usage Breakdown

The third line details the CPU utilization.

• us: Percentage of CPU time spent running user processes that are not niced.
• sy: Percentage of CPU time spent running the kernel and its processes.
• ni: Percentage of CPU time spent running niced (low priority) user processes.
• id: Percentage of CPU time that is idle.
• wa: Percentage of CPU time spent waiting for I/O operations to complete. A high value might indicate a disk or network bottleneck.
• hi: Percentage of CPU time spent servicing hardware interrupts.
• si: Percentage of CPU time spent servicing software interrupts.
• st: Steal time. In virtualized environments, this is the percentage of CPU time a virtual CPU waits for a real CPU, while the hypervisor is servicing another virtual processor.

Memory and Swap Information

The fourth and fifth lines show memory and swap space usage, respectively. This includes total, used, and free amounts.

The Process List

The main body of top is a list of the most resource-intensive processes.

• PID: The unique Process ID.
• USER: The user who owns the process.
• PR: The scheduling priority of the process.
• NI: The "nice" value, which affects its priority.
• VIRT: Virtual Memory used by the process. This is the total amount of memory the process can access.
• RES: Resident Memory used by the process. This is the non-swapped physical memory a task is using. Understanding the difference between linux top virt res is key for memory analysis.
• SHR: Shared Memory used by the process.
• S: The status of the process: S=sleep, R=running, Z=zombie, D=uninterruptible sleep, T=stopped.
• %CPU: The percentage of CPU time used by this process since the last update.
• %MEM: The percentage of physical RAM used by this process.
• TIME+: The total CPU time the process has used since it started.
• COMMAND: The command name or command line that started the process.

You can also monitor a specific process by its ID, which is useful for focused troubleshooting:

top -p 1